android ble connect slowly

Question

Hi I'm writing an Android app to connect to a BLE peripheral device. Android 4.4.2, Galaxy Nexus.

I have an LED on the device to indicate connection state.

The issue is the duration from connectGatt() call to the point of receiving onConnectionStateChange STATE_CONNECTED are so inconsistent. Some time it is very quick but most of the time it takes 5s or more. Turning Bluetooth off/on does not have any effect.

I tried TI BTool on PC with the TI BLE Dongle and it always establishes connection very fast.

I also tried with an iPhone 5S and it is fast too.

1. Any one has experienced this issue too?
2. Is there any chance we can improve this?

Answer 1

Passing true to connectGatt() autoconnect argument requests a background connection, while passing false requests a direct connection. BluetoothGatt#connect() always requests a background connection.

Background connection (according to Bluedroid sources from 4.4.2 AOSP) has scan interval of 1280ms and a window of 11.25ms. This corresponds to about 0.9% duty cycle which explains why connections, when not scanning, can take a long time to complete.

Direct connection has interval of 60ms and window of 30ms so connections complete much faster. Additionally there can only be one direct connection request pending at a time and it times out after 30 seconds. onConnectionStateChange() gets called with state=2, status=133 to indicate this timeout.

I have verified this behavior on Nexus5 but obviously YMMV.

I should mention that there is a race condition in BluetoothGatt.java that can cause a direct connection request even if autoconnect=true is passed into BluetoothDevice#connectGatt().